Many types of products are created or improved by electron beam (e-beam), or other high-energy radiation, processing. Products include appliance parts, gaskets, manifolds, tubing, electrical connectors, molded parts, resin pellets, etc. The possibilities are endless because, in many products, property improvements can be attained through e-beam processing. Commodity resins and polymeric materials including polyethylene, ethylene vinyl acetate, polyvinyl chloride, polyamides, and certain types of rubber, such as neoprene, silicone, and ethylene-propylene rubbers, can all realize substantial property enhancement. Property improvements of these polymeric materials may include an increase in tensile strength, impact strength, abrasion resistance, chemical resistance, heat deflection, modulus, hardness, service temperature, barrier properties, crack-resistance, creep resistance, and fatigue resistance. Other special attributes can be imparted to polymers, such as heat-shrink properties, positive temperature coefficient properties, and various other special properties achieved by crosslinking or scission of polymers. Often these property improvements are achieved by the e-beam processing of formed parts.
A number of patents have been granted for processing polymeric bulk material solids, such as polypropylene, and other polyolefin polymers, to improve melt strength, heat resistance, or other physical properties, including U.S. Pat. Nos. 4,916,198; 5,047,446; 5,047,485; 5,541,236; 5,554,668; 5,591,785; 5,605,936; and 5,731,362. In general, the processes described in these patents involve treatment of finely divided polymeric material, which is layered on a traveling belt in the required environment. The speed of the traveling belt is selected so that the layer of finely divided polymeric material passes through the electron beams at a rate to receive the desired dose of radiation. Other process steps may be involved such as treatment of the irradiated polymers in a fluidized bed with nitrogen or other inert gas. Expensive equipment is employed to process the polymeric particles in the environmentally-controlled zone.
Polymeric material solids and other material solids often require thermal treatment subsequent to irradiation. Heat treating irradiated material solids is commonly accomplished in an oven, a fluidized bed reactor, or other equipment using gas as the heat transfer fluid. A number of patents have been granted. For example, in U.S. Pat. No. 4,220,511, heat treatment in a gas-based oven promotes the desired chemical reaction with free radicals furthering the chain scission of the polymer. In U.S. Pat. No. 6,340,718, involving the irradiation of polytetrafluoroethylene, there is a post-irradiation cooling step which takes place in a closed loop pneumatic path, i.e. gas-based cooling step. In the case of high-melt strength polypropylene, the free radicals imparted by irradiation are quenched at elevated temperature in a fluid bed reactor using nitrogen as disclosed in U.S. Pat. Nos. 4,916,198 and 5,047,446. Other patents and applications relate to high-melt strength polypropylene, and envision the use of gas-based heat treatment methods as disclosed in U.S. Pat. Nos. 8,220,226 and 8,399,536.
In the case of high molecular weight polyethylene (HMWPE) which is treated with irradiation to improve the physical properties, the polymeric material is subsequently heated in an oven circulating air, nitrogen or, in some cases, a gas containing a reactant like acetylene. Undesirable free radicals remaining in HMWPE after the irradiation step are quenched by heat.
Thus, irradiation of high molecular weight solid organic polymers with high-energy radiation to improve the physical and mechanical properties, followed by additional processing, with a circulation oven or similar heating device are well developed in the patent art. Almost universally, the heat treatment methods specified in the prior art patents and implemented in commercial practice utilize hot air or other gas in an oven or a fluidized bed. The elimination of expensive equipment and the simplification of the processing steps currently used would be highly desirable.